To smoke marijuana in an enclosed vehicle, causing the vehicle to fill up with smoke. This achieves a better high because no smoke is wasted in the process, and is usually done with a blunt or a joint. This is known as clam baking.

"Me and George are going to clam bake in his car later, want to come?"
"We clam baked last night and we were trippin' balls after words."
"George clam baked in his car and now can't get the pot smell out of it."
"Hey, want to go clam baking later?"

Apparently, people from Connecticut call hot boxing clambaking. In the words of a certain CT resident “its smoking pot in your car, rolling the windows up...like a clambake.” It combines the wonders of first hand and second hand smoke to maximize THC intake.